Book excerpt: The purpose of the test is to measure the capacity of a chemical to activate or inhibit the transcription of a genetic construct (THb/ZIP-GFP X. laevis eleutheroembryos) either by binding to the thyroid hormone receptor or by modifying the amount of thyroid hormone available for transcription. The test is based on the use of a transgenic frog (possessing the genetic construction THb/ZIP-GFP) of the species X. laevis. GFP is a fluorescent protein that can be used to affirm the presence of the substance and to follow its action. This test is based on the quantification of fluorescence, and thus of the GFP protein, throughout the eleutheroembryo exposed to a test chemical. Indeed, if the fluorescence is > 12% by including the highest concentration of the test, the test is considered positive. Organisms are exposed in chemically inert cell culture plates (6 wells). Each well contains ten organisms in solution. In a run, 20 organisms are exposed to each test concentration. Tadpoles are exposed to the test chemical at NF stage 45 for a duration of 72 hours. A positive result of the XETA test indicates in vivo that a substance induces damage to the receptor and metabolization of the thyroid hormone. A negative result indicates that, under test conditions, the test substance does not induce detectable damage to the thyroid and its functioning.